CN107427257B

CN107427257B - Magnetic resonance imaging thermometry using proton resonance frequency and T1 measurements

Info

Publication number: CN107427257B
Application number: CN201680018452.XA
Authority: CN
Inventors: S·魏斯
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2015-03-26
Filing date: 2016-03-28
Publication date: 2021-07-13
Anticipated expiration: 2036-03-28
Also published as: EP3274733A1; US20180120396A1; WO2016151146A1; JP2018509230A; JP6820861B2; CN107427257A

Abstract

The invention provides a method of operating a medical instrument (100, 400, 500, 600) having a magnetic resonance imaging system (102). The method comprises the following steps: acquiring (202) equilibrium magnetization magnetic resonance imaging data (148) by controlling the magnetic resonance imaging system according to a T1 measurement magnetic resonance imaging protocol, and calculating an equilibrium magnetization baseline image (156). The method further comprises acquiring (206) dynamic PRFS magnetic resonance data according to a proton resonance frequency-shifted magnetic resonance imaging protocol. The method further includes repeatedly acquiring (208) a magnetic resonance data portion (152) in accordance with the T1 measurement magnetic resonance imaging protocol, the T1 measurement magnetic resonance imaging protocol having saturation preparation (804) at a beginning of the acquiring. The acquisition of the dynamic PRFS magnetic resonance data and the acquisition of the magnetic resonance data portions are interleaved. The method further includes repeatedly re-assembling (212) the magnetic resonance data portions into dynamic T1 magnetic resonance data. The method also includes repeatedly calculating (214) a T1 map (158) using the reassembled dynamic T1 magnetic resonance data and the balanced magnetization baseline image. The method further includes repeatedly calculating (216) a PRFS phase calibration (160) using the dynamic PRFS magnetic resonance data and the T1 map. The method further comprises calculating (218) a PRFS temperature map (162) using the dynamic PRFS magnetic resonance data and the PRFS phase calibration if the PRFS phase calibration has been calculated.

Description

Magnetic resonance imaging thermometry using proton resonance frequency and T1 measurements

Technical Field

The present invention relates to magnetic resonance imaging, and in particular to magnetic resonance imaging thermometry.

Background

Magnetic resonance thermometry may be used to determine the absolute temperature or change in temperature of a volume, depending on the technique used. To determine the absolute temperature, several magnetic resonance peaks are typically measured. Methods of measuring changes in temperature are generally faster and have been used to make temperature measurements for guiding thermal treatment. For example, proton resonance frequency shift (PRFS or PRF) based MR thermometry can be used to provide a temperature map quickly and accurately. However, PRFS-based methods rely on making accurate phase calibrations, which in turn are very susceptible to variations in the B0 field of the magnet.

The journal article "Hybrid proton response frequency/T1technique for simultaneous electron temperature monitoring in applications and requests properties" in Magn Reson Med, 69:62-70 doi:10.1002/mrm.24228(2013) describes a combined T1 and PRFS pulse sequence, where the standard RF spoiled gradient echo sequence is run in a dynamic mode with two flip angles staggered with each time frame.

Disclosure of Invention

The invention provides a medical instrument, a method of operating a medical instrument, and a computer program product in the independent claims. Embodiments are given in the dependent claims.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as an apparatus, method, or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit," module "or" system. Furthermore, aspects of the present invention may take the form of a computer program product embodied on one or more computer-readable media having computer-executable code embodied thereon.

Any combination of one or more computer-readable media may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. 'computer-readable storage medium' as used herein encompasses any tangible storage medium that can store instructions executable by a processor of a computing device. The computer-readable storage medium may be referred to as a computer-readable non-transitory storage medium. Computer readable storage media may also be referred to as tangible computer readable media. In some embodiments, the computer-readable storage medium may also be capable of storing data that is accessible by a processor of the computing device. Examples of computer readable storage media include, but are not limited to: a floppy disk, a magnetic hard drive, a solid state disk, flash memory, a USB thumb drive, Random Access Memory (RAM), Read Only Memory (ROM), an optical disk, a magneto-optical disk, and a register file for a processor. Examples of optical disks include Compact Disks (CDs), Digital Versatile Disks (DVDs), and Blu-ray disks (BDs), such as CD-ROMs, CD-RWs, CD-R, DVD-ROMs, DVD-RWs, DVD-R, BD-Rs, or BD-RE disks. The term computer readable storage medium also refers to various types of recording media that can be accessed by a computer device via a network or a communication link. For example, data may be retrieved over a modem, over the internet, or over a local area network. Computer executable code embodied on a computer readable medium may be transmitted using any suitable medium, including but not limited to: wireless, wired, fiber optic cable, RF, etc., or any suitable combination of the foregoing.

A computer readable signal medium may include, for example, a propagated data signal with computer executable code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to: electromagnetic, optical, or any suitable combination thereof. The computer readable signal medium may be any computer readable medium that: the computer readable medium is not a computer readable storage medium and it can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device.

'computer memory' or 'memory' is an example of a computer-readable storage medium. Computer memory is any memory that can be directly accessed by a processor. 'computer storage' or 'storage' is another example of a computer-readable storage medium. Computer storage is any non-volatile computer-readable storage medium. In some embodiments, the computer storage may also be computer memory, or vice versa.

A 'processor' as used herein encompasses an electronic component capable of executing a program or machine-executable instructions or computer-executable code. References to a computing device that includes a "processor" should be interpreted as potentially containing more than one processor or processing core. The processor may be, for example, a multi-core processor. A processor may also refer to a collection of processors within a single computer system or distributed among multiple computer systems. The term computing device should also be read to possibly refer to a collection or network of computing devices, each of which includes one or more processors. The computer executable code may be executed by multiple processors, which may be within the same computing device or even distributed across multiple computing devices.

The computer executable code may include machine executable instructions or programs that cause a processor to perform an aspect of the present invention. Computer executable code for performing operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language (e.g., Java, Smalltalk, C + +, etc.) and a conventional flow programming language (e.g., the "C" programming language or similar programming languages), and compiled as machine executable instructions. In some instances, the computer executable code may be in a high level language or in a pre-compiled form, and may be used in conjunction with an interpreter that generates machine executable instructions online.

The computer executable code may run entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a Local Area Network (LAN) or a Wide Area Network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet service provider).

Aspects of the present invention are described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block or portion of the blocks of the flowchart, illustrations and/or block diagrams can be implemented by computer program instructions in computer-executable code where appropriate. It will also be understood that blocks of the various flow diagrams, illustrations, and/or block diagrams, when not mutually exclusive, may be combined. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

A 'user interface' as used herein is an interface that allows a user or operator to interact with a computer or computer system. The 'user interface' may also be referred to as a 'human interface device'. The user interface may provide information or data to and/or receive information or data from an operator. The user interface may enable input from an operator to be received by the computer and may provide output from the computer to a user. In other words, the user interface may allow an operator to control or manipulate the computer, and the interface may allow the computer to indicate the effect of the operator's control or manipulation. The display of data or information on a display or graphical user interface is an example of providing information to an operator. Receiving data through a keyboard, mouse, trackball, trackpad, pointing stick, tablet, joystick, gamepad, webcam, head-mounted device, gear lever, steering wheel, foot pedal, wired gloves, dance mat, remote control, and accelerometer are all examples of user interface components that enable receiving information or data from an operator.

'hardware interface' as used herein encompasses an interface that enables a processor of a computer system to interact with and/or control an external computing device and/or apparatus. The hardware interface may allow the processor to send control signals or instructions to an external computing device and/or apparatus. The hardware interface may also enable the processor to exchange data with external computing devices and/or apparatus. Examples of hardware interfaces include, but are not limited to: a universal serial bus, an IEEE 1394 port, a parallel port, an IEEE 1284 port, a serial port, an RS-232 port, an IEEE-488 port, a Bluetooth connection, a wireless local area network connection, a TCP/IP connection, an Ethernet connection, a control voltage interface, a MIDI interface, an analog input interface, and a digital input interface.

A 'display' or 'display device' as used herein encompasses an output device or user interface suitable for displaying images or data. The display may output visual, auditory, and/or tactile data. Examples of displays include, but are not limited to: computer monitors, television screens, touch screens, tactile electronic displays, braille screens, Cathode Ray Tubes (CRTs), memory tubes, bi-stable displays, electronic paper, vector displays, flat panel displays, vacuum fluorescent displays (VFs), Light Emitting Diode (LED) displays, electroluminescent displays (ELDs), Plasma Display Panels (PDPs), Liquid Crystal Displays (LCDs), organic light emitting diode displays (OLEDs), projectors, and head mounted displays.

Medical image data is defined herein as two-dimensional data or three-dimensional data that has been acquired using a medical imaging scanner. A medical imaging scanner is defined herein as a device adapted to acquire information about the body structure of a patient and to construct two-dimensional medical image data or a set of three-dimensional medical image data. Medical image data can be used to construct visualizations useful for the diagnosis of physicians. The visualization can be performed using a computer.

Magnetic Resonance (MR) data is defined herein as being the recorded measurements of radio frequency signals emitted by atomic spins by an antenna of a magnetic resonance apparatus during a magnetic resonance imaging scan. Magnetic resonance data is an example of medical image data. A Magnetic Resonance Imaging (MRI) image is defined herein as being a two-dimensional visualization or a three-dimensional visualization reconstructed of anatomical data contained within the magnetic resonance imaging data. The visualization can be performed using a computer.

The magnetic resonance data may comprise measurements of radio frequency signals emitted by atomic spins by an antenna of the magnetic resonance apparatus during magnetic resonance imaging, which magnetic resonance data contains information that may be used for magnetic resonance thermometry. Magnetic resonance thermometry works by measuring changes in temperature sensitive parameters. Examples of parameters that can be measured during magnetic resonance thermometry are: proton resonance frequency shift, diffusion coefficient, or change in T1 and/or T2 relaxation times may be used to measure temperature using magnetic resonance. Proton resonance frequency shift is temperature dependent because the magnetic field experienced by individual protons, hydrogen atoms, depends on the surrounding molecular structure. The increase in temperature reduces the molecular sieve due to the temperature affecting hydrogen bonding. This causes a temperature dependence of the proton resonance frequency.

The proton density depends linearly on the equilibrium magnetization. Thus, the proton density weighted image can be used to determine the temperature variation.

Relaxation times T1, T2, and T2 stars (sometimes written as T2) are also temperature dependent. Therefore, reconstruction of T1, T2, and T2 star weighted images can be used to construct a thermal or temperature map.

Temperature also affects the brownian motion of molecules in aqueous solution. Thus, a pulse sequence capable of measuring diffusion coefficients (e.g., pulsed diffusion gradient spin echo) can be used to measure temperature.

One of the most useful methods of measuring temperature using magnetic resonance is to measure the Proton Resonance Frequency (PRF) shift of water protons. The resonance frequency of the protons is temperature dependent. The frequency shift will change the measured phase of the water protons due to temperature changes in the voxels. Thus, the temperature change between the two phase images can be determined. This method of determining the temperature has the following advantages: i.e. it is relatively fast compared to other methods.

As used herein, an 'ultrasonic window' encompasses a window that is effectively transparent to ultrasonic waves or ultrasonic energy. Typically, a thin film or membrane is used as the ultrasound window. The ultrasound window may be made, for example, from a thin film of BoPET (biaxially oriented polyethylene terephthalate).

In one aspect, the present invention provides a medical instrument. The medical instrument comprises a magnetic resonance imaging system for acquiring magnetic resonance data within an imaging zone. The medical instrument further includes a memory storing machine executable instructions. The memory also stores a first pulse sequence command, a second pulse sequence command, and a third pulse sequence command. As used herein, pulse sequence commands are either commands that can be used to directly control the magnetic resonance imaging system or data that can be converted into such commands. For example, pulse sequences are typically defined in terms of timing diagrams. The data used to define the timing diagrams may be converted into commands for controlling the magnetic resonance imaging system. The pulse sequence commands may also control data for controlling the operation of other instruments that may be used in conjunction with the magnetic resonance imaging system. For example, the pulse sequence commands may also include commands for controlling a temperature control system.

The first pulse sequence commands cause the magnetic resonance imaging system to acquire equilibrium magnetization magnetic resonance data in accordance with the T1 measurement magnetic resonance imaging protocol. The term equilibrium magnetization magnetic resonance data is a label that: which refers to the specific magnetic resonance data acquired according to the T1 measurement magnetic resonance imaging protocol. The second pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic PRFS magnetic resonance data according to a proton dwell frequency shift magnetic resonance imaging protocol. The abbreviation PRFS is used as abbreviation for proton resonance frequency shift. The third pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic T1 magnetic resonance data in accordance with the T1 measurement magnetic resonance imaging protocol. The third pulse sequence commands also cause the magnetic resonance imaging system to sequentially acquire T1 magnetic resonance data as a set of magnetic resonance data portions. The first pulse sequence commands cause the entire k-space of the T1 magnetic resonance imaging protocol to be acquired at once. The third pulse sequence commands cause the acquisition of k-space data in the magnetic resonance data portion. For example, a trajectory in k-space may be delineated for each of the acquired magnetic resonance data portions.

The medical instrument further comprises a processor for controlling the medical instrument. The processor may also be considered a controller, for example. Execution of the machine-executable instructions causes the processor to acquire equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands. The T1 measurement magnetic resonance imaging protocol is a magnetic resonance imaging protocol for measuring T1 values for each voxel being imaged. Execution of the machine-executable instructions further causes the processor to calculate an equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data. For each voxel imaged, the equilibrium magnetization is calculated, and this can be represented as a set of values in a two-dimensional array or a three-dimensional array of images or data.

Execution of the machine-executable instructions further cause the processor to repeatedly acquire the PRFS magnetic resonance data by controlling the magnetic resonance imaging system with the second pulse sequence commands. Execution of the machine-executable instructions further causes the processor to repeatedly acquire a magnetic resonance data portion by controlling the magnetic resonance imaging system with the third pulse sequence commands. The acquisition of the PRFS magnetic resonance data and the acquisition of the magnetic resonance data portions are interleaved. In other words, the PRFS magnetic resonance data is acquired interleaved with acquiring a portion of the set of magnetic resonance data portions. The magnetic resonance data part belongs to a set of magnetic resonance data parts.

Execution of the machine-executable instructions further causes the processor to repeatedly reassemble the set of magnetic resonance data portions into dynamic T1 magnetic resonance data after the complete set of magnetic resonance data portions is acquired. Since the PRFS magnetic resonance data and the magnetic resonance data portions are actually interleaved, a complete set of magnetic resonance data portions will be acquired. At this point, they are reassembled into full dynamic T1 magnetic resonance data. Execution of the machine-executable instructions further causes the processor to calculate a T1 map using the reassembled dynamic T1 magnetic resonance data with the equilibrium magnetization image. The computation of the T1 map is achieved by using the balanced magnetization image. The equilibrium magnetization image is reconstructed from data acquired before the first PRFS magnetic resonance data was acquired.

Thus, the acquisition of the PRFS magnetic resonance data does not interfere with the measurement of the balanced magnetization baseline image. Execution of the machine-executable instructions further cause the processor to calculate a PRFS phase calibration using the PRFS magnetic resonance data and the T1 map. The proton resonance frequency shift method of measuring temperature is very fast and accurate; however, this is susceptible to B0 drift. However, B0 drift is a condition that occurs on a time scale comparable to the acquisition of magnetic resonance data. By repeatedly acquiring T1 magnetic resonance data during the acquisition of normal PRFS magnetic resonance data, the re-assembled magnetic resonance data portions can be used to periodically recalibrate the PRFS method. Execution of the machine-executable instructions further cause the processor to calculate a PRFS temperature map using the PRFS magnetic resonance data and the PRFS phase calibration if the PRFS phase calibration has been calculated. In other examples, the first PRFS phase calibration is calculated by using the first PRFS magnetic resonance data. For example, a temperature distribution within the body may be assumed and used to calculate an initial calibration. For example, first dynamic PRFS magnetic resonance data that can be acquired before performing actual heating of the tissue can be used for the initial PRFS phase calibration. In other examples, the T1 data and baseline magnetization from the equilibrium magnetization baseline image may be used to calculate a temperature from the T1 value, which is then used to initially calculate a PRFS phase calibration.

This embodiment may be beneficial because it provides a stable way of measuring the temperature of a subject for magnetic resonance imaging using the PRFS method of measuring temperature.

In another embodiment, execution of the machine-executable instructions may further cause the processor to display a PRFS temperature map on a display to store the PRFS temperature map in a computer storage device or to transmit the PRFS temperature map to another computer system via a network or other data transmission system.

In another embodiment execution of the instructions further cause the processor to repeatedly calculate a dynamic image from selected magnetic resonance data selected from the set of magnetic resonance data portions. The selected magnetic resonance data is selected such that the selected magnetic resonance data portion maximizes longitudinal magnetization. Each time a magnetic resonance data portion is acquired, a measurement of a different delay from saturation preparation is obtained. The recovery of the longitudinal magnetization requires more time in the data collected further in time from the saturation preparation. After the entire set of magnetic resonance data portions has been acquired and combined. The selected magnetic resonance data can be removed from each of the magnetic resonance data portions. These data are chosen such that only those portions that maximize longitudinal magnetization are selected (i.e., data acquired with the maximum delay from saturation preparation). The selection of this data results in a dynamic image with longitudinal magnetization close to the equilibrium value. This in turn enables a direct comparison between the dynamic image and the equilibrium magnetization baseline image.

Execution of the machine-executable instructions further causes the processor to repeatedly detect object motion above a predetermined threshold using the balanced magnetization baseline image and the dynamic image. The magnetic resonance data portion is only a portion of a complete k-space; however, a complete set of magnetic resonance data portions can be used to reconstruct such an image: the image may be compared to the balanced magnetization baseline image.

In another embodiment, the dynamic image is calculated from late (late) acquired data in the T1 relaxation curve, i.e. from data at the end of the data portion. Late in the T1 relaxation curve means herein that the portion of the magnetic resonance data selected to reconstruct the dynamic image is the last acquired or one of the last several acquired. Longitudinal magnetization has the opportunity to substantially recover, which enables direct comparison of the dynamic image with the equilibrium magnetization baseline image. The balanced magnetization image may also be referred to herein as M₀And (4) an image.

Calculating a dynamic image from later acquired data in the T1 relaxation curve may be advantageous because the contrast of the resulting dynamic image will be similar to the contrast of the equilibrium image. Various techniques known in the art may be used to detect whether the object has moved beyond a predetermined threshold when the equilibrium magnetization baseline image is acquired.

This enables a fast detection of the motion of the object, since the magnetic resonance data portions are acquired relatively frequently. This may enable a quick determination of whether the PRFS temperature map is no longer valid. Execution of the machine-executable instructions further causes the processor to reacquire equilibrium magnetization magnetic resonance data by controlling the magnetic resonance imaging system using the first pulse sequence commands if motion of the subject is detected. In some instances, this may require suspending acquisition of the magnetic resonance data together in order to enable the equilibrium magnetization to return to its equilibrium state. Execution of the machine-executable instructions further cause the processor to repeatedly recalculate the equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data if motion of a subject is detected. In some instances, this step may also involve calculating a new PRFS phase calibration. This embodiment may have the benefit of: that is, the motion of the object can be detected quickly, and the correction of the PRFS temperature map can be performed. This may result in a more accurate PRFS temperature map.

In another embodiment, object motion is detected using a cross-correlation algorithm.

In another embodiment, rigid body motion detection algorithms are used to detect object motion.

In another embodiment, an elastic registration algorithm is used to detect object motion.

In another embodiment, object motion is detected using an optical flow algorithm.

In another embodiment, the medical instrument further comprises a temperature control system for changing the temperature within the target region. The target region is within the imaging region.

In another embodiment, the temperature control system is a high intensity focused ultrasound system.

In another embodiment, the temperature control system is a radiofrequency tissue heating system.

In another embodiment, the temperature control system is a microwave applicator.

In another embodiment, the temperature control system is a cryoablator.

In another embodiment, the temperature control system is a laser.

In another embodiment, execution of the machine-executable instructions further causes the processor to receive a temperature control system command that causes the temperature control system to change the temperature of the target zone. The temperature control system command may be a command used by the processor to directly control the temperature control system, or the temperature control system command may be a command or data used to generate a command used to control the temperature control system. Execution of the machine-executable instructions further cause the processor to repeatedly change the temperature control system command using the PRFS temperature map. These steps effectively form a control loop for controlling the temperature control system. For example, the temperature control system commands may specify an area or a specific location within the object to be heated to a specific temperature for a duration of time. The PRFS temperature map can be used as feedback to accurately control the temperature control system to follow the temperature control system commands.

In another embodiment, the medical instrument further comprises a user interface having a display. Execution of the machine-executable instructions further causes the processor to display the PRFS temperature map on the display. Execution of the machine-executable instructions further causes the processor to receive control data from the user interface. The control data may for example comprise commands to heat or not heat a specific area of the object. The user control data may also include data that changes the behavior of the temperature control system. Execution of the machine-executable instructions further causes the processor to use the user control data to change the temperature control system command.

In another embodiment, execution of the machine-executable instructions further causes the processor to control the temperature control system with the temperature control system commands.

In another embodiment, the T1 magnetic resonance imaging protocol is a saturation recovery look-locker magnetic resonance imaging protocol.

In another embodiment, operation of the magnetic resonance imaging system causes the processor to perform the following after a predetermined time interval: reacquiring the equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands, and recalculating an equilibrium magnetization image from the equilibrium magnetization magnetic resonance imaging data if motion of the subject is detected. In some examples, the PRFS phase calibration may also be recalculated. In this example, the equilibrium magnetization image is acquired and calculated after a period of time has elapsed. Even if, for example, object motion has not been detected, this is still beneficial to periodically check to ensure that the equilibrium magnetization image is still accurate.

In another embodiment the third pulse sequence commands cause the magnetic resonance imaging system to perform saturation preparation at the beginning of the acquisition of each magnetic resonance data portion. Saturation preparation as used herein encompasses radio frequency pulses and gradient pulses that reduce longitudinal magnetization to zero and destroy all transverse magnetization. In this document, the saturation preparation is sometimes referred to as a "saturation preparation pulse".

This may be beneficial because the saturated radio frequency preparation reduces the longitudinal magnetization to zero, which effectively negates the effect of performing the PRFS measurement immediately prior to taking the T1 measurement.

In another aspect, the present invention provides a method of operating a medical instrument. The medical instrument comprises a magnetic resonance imaging system for acquiring magnetic resonance data within an imaging zone. The method comprises the step of acquiring equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using first pulse sequence commands. The first pulse sequence commands cause the magnetic resonance imaging system to acquire equilibrium magnetization magnetic resonance data according to a T1 measurement magnetic resonance imaging protocol. The method further includes calculating an equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data.

The method further comprises repeatedly acquiring PRFS magnetic resonance data by controlling the magnetic resonance imaging system with second pulse sequence commands. The second pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic PRFS magnetic resonance data according to a proton resonance frequency-shifted magnetic resonance shaping protocol. The method further comprises repeatedly acquiring magnetic resonance data portions by controlling the magnetic resonance imaging system with third pulse sequence commands. The third pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic T1 magnetic resonance data in accordance with a T1 measurement magnetic resonance imaging protocol.

The third pulse sequence commands also cause the magnetic resonance imaging system to sequentially acquire the dynamic T1 magnetic resonance data as a set of magnetic resonance data portions. The acquisition of the dynamic PRFS magnetic resonance data and the acquisition of the magnetic resonance data portions are interleaved. The magnetic resonance data part belongs to a set of magnetic resonance data parts. The method further comprises repeatedly reassembling the set of magnetic resonance data portions into dynamic T1 magnetic resonance data after the complete set of magnetic resonance data portions is acquired. The method also includes calculating a T1 map using the reassembled dynamic T1 magnetic resonance data and the balanced magnetization baseline image.

The method further includes repeatedly calculating a PRFS phase calibration using the dynamic PRFS magnetic resonance data and the T1 map. The method further comprises calculating a PRFS temperature map using the PRFS magnetic resonance data and the PRFS phase calibration if the PRFS phase calibration has been calculated.

The method further comprises repeatedly computing dynamic images from the magnetic resonance data portions. This is done after each magnetic resonance data portion is acquired. The method further includes repeatedly detecting object motion above a predetermined threshold using the balanced magnetization baseline image and the dynamic image. The method further comprises repeatedly reacquiring equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using first pulse sequence commands in case motion of the object is detected. The method further includes repeatedly recalculating the equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data if subject motion is detected.

In another embodiment, the method further comprises correcting for motion that may occur between the M0 scan (zero magnetization scan) and the dynamic T1 acquisition by comparing each last image acquired during the T1 relaxation after saturation to the equilibrium magnetization baseline image and by reacquiring the equilibrium magnetization data after suspending the dynamic acquisition if the motion exceeds a user-defined limit.

In another aspect, the invention provides a computer program product comprising machine executable instructions for execution by a processor controlling a medical instrument. The medical instrument comprises a magnetic resonance imaging system for acquiring magnetic resonance data within an imaging zone. Execution of the machine-executable instructions causes the processor to acquire equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands. The first pulse sequence commands cause the magnetic resonance imaging system to acquire the equilibrium magnetization magnetic resonance data according to a T1 measurement magnetic resonance imaging protocol.

Execution of the machine-executable instructions further causes the processor to calculate an equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data. Execution of the machine-executable instructions causes the processor to repeatedly acquire dynamic PRFS magnetic resonance data by controlling the magnetic resonance imaging system with the second pulse sequence commands. The second pulse sequence commands cause the magnetic resonance imaging system to acquire the dynamic PRFS magnetic resonance data according to a proton resonance frequency-shifted magnetic resonance imaging protocol. Execution of the machine-executable instructions further causes the processor to repeatedly acquire a magnetic resonance data portion by controlling the magnetic resonance imaging system with the third pulse sequence commands. The third pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic T1 magnetic resonance data in accordance with a T1 measurement magnetic resonance imaging protocol. The third pulse sequence commands also cause the magnetic resonance imaging system to sequentially acquire the dynamic T1 magnetic resonance data as a set of magnetic resonance data portions. The acquisition of the dynamic PRFS magnetic resonance data and the acquisition of the magnetic resonance data portions are interleaved. The magnetic resonance data part belongs to a set of magnetic resonance data parts.

Execution of the machine-executable instructions further causes the processor to repeatedly reassemble the set of magnetic resonance data portions into dynamic T1 magnetic resonance data after the complete set of magnetic resonance data portions is acquired. Execution of the machine-executable instructions further cause the processor to repeatedly calculate a T1 map using the reassembled dynamic T1 magnetic resonance data and the equilibrium magnetization image. Execution of the machine-executable instructions further cause the processor to repeatedly calculate a PRFS phase calibration using the dynamic PRFS magnetic resonance data and the T1 map. Execution of the machine-executable instructions further cause the processor to calculate a PRFS temperature map using the dynamic PRFS magnetic resonance data and the PRFS phase calibration if the PRFS phase calibration has been calculated.

It should be understood that one or more of the foregoing embodiments of the invention may be combined, as long as the combined embodiments are not mutually exclusive.

Drawings

In the following, preferred embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings, in which:

fig. 1 illustrates an example of a medical instrument;

FIG. 2 shows a flow chart illustrating a method of operating the medical instrument of FIG. 1, FIG. 4, FIG. 5, or FIG. 6;

FIG. 3 shows a flow chart illustrating a further method of operating the medical instrument of FIG. 1, FIG. 4, FIG. 5, or FIG. 6;

fig. 4 illustrates a further example of a medical instrument;

fig. 5 illustrates a further example of a medical instrument;

fig. 6 illustrates a further example of a medical instrument;

FIG. 7 shows a flow chart illustrating a further method of operating the medical instrument of FIG. 4, FIG. 5, or FIG. 6;

FIG. 8 illustrates a combined pulse sequence protocol; and is

Fig. 9 illustrates k-space sampling for a first pulse sequence command and a second pulse sequence command.

List of reference numerals

100 medical instrument

102 magnetic resonance imaging system

104 magnet

106 magnet bore

108 imaging zone

110 magnetic field gradient coil

112 magnetic field gradient coil power supply

114 radio frequency coil

116 radio frequency transceiver

118 object

120 object support

122 computer system

124 hardware interface

126 processor

128 user interface

130 computer storage device

132 computer memory

140 first pulse sequence command

142 second pulse sequence commands

144 third pulse train command

148 balanced magnetization magnetic resonance data

150 dynamic PRFS magnetic resonance data

152 magnetic resonance data portion

154 dynamic T1 magnetic resonance data reassembled

156 balanced magnetization baseline image

158T 1 diagram

160 PRFS phase calibration

162 PRFS temperature map

170 control module

172 image reconstruction module

174 image processing module

176 temperature drawing module

202 acquire equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using first pulse sequence commands

The balanced magnetization baseline image is computed 204 from the balanced magnetization magnetic resonance imaging data.

206 acquire PRFS magnetic resonance data by controlling the magnetic resonance imaging system with second pulse sequence commands

208 acquire a magnetic resonance data portion by controlling the magnetic resonance imaging system with the third pulse sequence commands

210 acquired all data portions of the dynamic T1 magnetic resonance data?

212 reassemble the set of magnetic resonance data portions into dynamic T1 magnetic resonance data after acquiring the complete set of magnetic resonance data portions

214 use the reassembled dynamic T1 magnetic resonance data and the balanced magnetization image to compute a T1 map

216 use PRFS magnetic resonance data and T1 maps to compute PRFS phase calibration

218 calculate a PRFS temperature map using PRFS magnetic resonance data and PRFS phase calibration with PRFS phase calibration having been calculated

220 protocol complete?

300 computing a dynamic image from a magnetic resonance data portion

302 detected motion?

303 pause

304 reacquires equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands in the event that motion of the subject is detected

306 recalculating the equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data if subject motion is detected

400 medical device

402 high intensity focused ultrasound system

404 fluid-filled chamber

406 ultrasonic transducer

408 mechanism

410 mechanical actuator/power supply

412 path of ultrasound

414 ultrasonic window

416 gel pad

418 sonication points

420 target area

430 dynamic image

432 temperature control system commands

440 motion detection module

442 temperature control system command modification module

500 medical device

501 radiofrequency tissue heating system

502 antenna

504 radio frequency transmitter

600 medical device

601 thermal disposal system

602 applicator

604 supply system

700 receive a temperature control system command that causes the temperature control system to alter the temperature of the target area

702 controls the temperature control system with temperature control system commands 706 to modify the temperature control system commands using the PRFS temperature map

800 combined pulse sequence

Graphical representation of acquisition of 802 dynamic T1 data

804 saturation preparation

806 data acquisition

808 space coding

900 balanced k-space ordering of magnetic resonance data

K-space order of magnetic resonance data portions of 902 dynamic T1 magnetic resonance data

Detailed Description

In the drawings, like numbered elements are either equivalent elements or perform the same function. If the functions are equivalent, it will not be necessary to discuss elements in later figures that have been previously discussed.

Fig. 1 illustrates an example of a medical instrument. The medical instrument 100 comprises a magnetic resonance imaging system 102. The magnetic resonance imaging system 102 is shown as including a magnet 104. Magnet 104 is a cylindrical superconducting magnet having a bore 106 through its center. Magnet 104 has a liquid helium cooled cryostat including superconducting coils. Permanent or normally conducting magnets may also be used. It is also possible to use different types of magnets, for example, both split cylindrical magnets and so-called open magnets can also be used. The split cylindrical magnet is similar to a standard cylindrical magnet except that the cryostat is split into two parts to allow access to the iso-plane of the magnet, such a magnet may be used, for example, in conjunction with charged particle beam therapy. An open magnet has two magnet parts, one above the other, with a space between them large enough to accommodate an object: the arrangement of the two part-regions is similar to that of a helmholtz coil. Open magnets are popular because objects are less restricted. Inside the cryostat of the cylindrical magnet is a collection of superconducting coils. Within the bore of the cylindrical magnet there is an imaging zone 108 in which the magnetic field is strong and uniform enough to perform magnetic resonance imaging.

Also within the bore of the magnet are magnetic field gradient coils 110, which magnetic field gradient coils 110 are used to acquire magnetic resonance data to spatially encode magnetic spins within the imaging zone of the magnet. The magnetic field gradient coils 110 are connected to a magnetic field gradient coil power supply 112. Magnetic field gradient coils are representative. Typically, the magnetic field gradient coils contain three separate sets of coils for spatial encoding in three orthogonal spatial directions. Magnetic field gradient power supply 112 supplies current to the magnetic field gradient coils. The current supplied to the field coils is controlled as a function of time and may be ramped and/or pulsed.

Adjacent to the imaging zone 108 is a radio frequency coil 114. The radio frequency coil 114 is connected to a radio frequency transceiver 116. Also within the bore of the magnet 106 is a subject 118 lying on a subject support 120 and partially within the imaging zone 108.

Adjacent to the imaging zone 108 is a radio frequency coil 114, the radio frequency coil 114 for manipulating the orientation of magnetic spins within the imaging zone 108 and for receiving radio transmissions from spins also within the imaging zone 108. The radio frequency coil 114 may include a plurality of coil elements. The radio frequency coil 114 may also be referred to as a channel or antenna. The radio frequency coil is connected to a radio frequency transceiver 116. The radio frequency coil 114 and the radio frequency transceiver 116 may be replaced by separate transmit and receive coils and separate transmitters and receivers. It should be understood that the radio frequency coil 114 and the radio frequency transceiver 116 are representative. The radio frequency coil 114 is intended to also represent a dedicated transmit antenna and a dedicated receive antenna. Similarly, the transceiver 116 may also represent a separate transmitter and a separate receiver.

The magnetic field gradient coil power supply 112 and the radio frequency transceiver 116 are connected to a hardware interface 124 of the computer system 122. The computer system 122 also includes a processor 126. The processor 126 is connected to the hardware interface 124. The hardware interface 124 enables the processor 126 to send data and commands to the magnetic resonance imaging system 102 and to receive data and commands from the magnetic resonance imaging system 102. Computer system 122 also includes a user interface 128, computer storage 130, and computer memory 132.

The computer storage 130 is shown as containing a first pulse sequence command 140, a second pulse sequence command 142, and a third pulse sequence command 144. The first pulse sequence commands 140 cause the magnetic resonance imaging system to acquire equilibrium magnetization magnetic resonance data in accordance with the T1 measurement magnetic resonance imaging protocol. The second pulse sequence commands 142 cause the magnetic resonance imaging system to acquire dynamic PRFS magnetic resonance data according to a proton resonance frequency-shifted magnetic resonance imaging protocol. The third pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic T1 magnetic resonance data 154 in accordance with a T1 measurement magnetic resonance imaging protocol. The third pulse sequence commands 144 also cause the magnetic resonance imaging system 102 to sequentially acquire dynamic T1 magnetic resonance data 154 as a set of magnetic resonance data portions 152. The computer storage 130 is further shown as containing equilibrium magnetization magnetic resonance data 148 acquired by controlling the magnetic resonance imaging system 102 with the first pulse sequence commands 140. The computer storage 130 is further shown as containing dynamic PRFS magnetic resonance data 150 acquired by controlling the magnetic resonance imaging system 102 with second pulse sequence commands 142. The computer storage 130 is further shown as containing a magnetic resonance data portion 152 acquired by controlling the magnetic resonance imaging system 102 with the third pulse sequence commands 144. The computer storage 130 is also shown as containing the reassembled dynamic T1 magnetic resonance data 154 assembled from the sequentially acquired magnetic resonance data portions 152. The computer storage 130 is also shown as containing a balanced magnetization baseline image 156 reconstructed from the balanced magnetization magnetic resonance data 148. The computer storage 130 is also shown as containing a T1 map 158 reconstructed from the balanced magnetization baseline image 156 and the reassembled dynamic T1 magnetic resonance data 154. The computer storage 130 is also shown as containing a PRFS phase calibration 160 calculated from the T1 map 158 and the dynamic PRFS magnetic resonance data 150. The computer storage 130 is also shown to contain a PRFS temperature map 162 calculated using a PRFS phase calibration 160 and later acquired dynamic PRFS magnetic resonance data 150.

The computer memory 132 is shown as containing a control module 170. Control module 170 includes computer-executable instructions that enable processor 126 to control the operation and function of medical instrument 100. The computer memory 132 is also shown as containing an image reconstruction module 172, the image reconstruction module 172 enabling the processor 126 to process the various

magnetic resonance data

148, 150, 152, 154 into various images or maps 156, 158, 160, 162. The computer memory 132 is also shown as containing an image processing module 174, which image processing module 174 enables the processor 126 to operate on or perform calculations on various images or figures. The computer memory 132 is also shown as containing a temperature mapping module 176. The temperature mapping module causes the processor 126 to apply a T1 temperature mapping technique and/or a PRFS temperature mapping technique. The contents of computer storage 130 and computer memory 132 may be duplicated from each other or the contents of computer storage 130 and computer memory 132 may be exchanged.

Fig. 2 shows a flow chart illustrating an example of a method of operating the medical instrument 100 of fig. 1. First, in step 202, the processor 126 controls the magnetic resonance imaging system 102 to acquire the equilibrium magnetization magnetic resonance data 148 by controlling the magnetic resonance imaging system 102 with the first pulse sequence commands 140. Next, in step 204, an equilibrium magnetization baseline image 156 is calculated from the equilibrium magnetization magnetic resonance imaging data 148. Next, in step 206, dynamic PRFS magnetic resonance data 150 is acquired by controlling the magnetic resonance imaging system 102 with the second pulse sequence commands 142. In step 208, a magnetic resonance data portion is acquired by controlling the magnetic resonance imaging system 102 with the third pulse sequence data 144. The acquisition of the PRFS magnetic resonance data 150 and the acquisition of the magnetic resonance data portions 152 are interleaved. Step 210 is a decision block. The problem for step 210 is the complete set of acquired magnetic resonance data portions. If not, the method proceeds to step 218 as described below. If the answer is yes, the method proceeds to step 212. In step 212, the set of magnetic resonance data portions is reassembled into the dynamic T1 magnetic resonance data 154.

Next, in step 214, a T1 map is computed using the reassembled dynamic T1 magnetic resonance data 154 and the balanced magnetization baseline image 156. Next, in step 216, PRFS phase calibration is calculated using the PRFS magnetic resonance data and the T1 map 154. Step 216 describes how to periodically replace or recalibrate the PRFS phase calibration using the T1 map and the balanced magnetization baseline image. There are various different ways in which the initial PRFS phase calibration may be performed. In other examples, the first time to acquire dynamic PRFS magnetic resonance data is used for the calibration. The initial calibration of the PRFS method is well known and therefore not discussed in detail here. A number of variations on how the PRFS phase calibration is initially calculated can be performed by making slight modifications to the methods described herein. The flow chart in fig. 2 is intended to illustrate how the computed T1 map can be used to periodically update the PRFS phase calibration.

In step 218, a PRFS temperature map is calculated. After step 218, the method proceeds to step 220, step 220 being another decision block. In step 220, the question is that the protocol is complete. If the answer is yes, the method proceeds to step 222, which is the end of the protocol, step 222. If the answer to this question is no, the method returns to step 206 and in step 206 the acquisition of interleaved PRFS magnetic resonance data and magnetic resonance data portions is started again. Also illustrated includes query box 220. The method in fig. 2 can be modified by breaking the workflow at any time to end the flow. The inclusion of step 220 and step 222 is intended to be illustrative only.

Fig. 3 shows a flow chart illustrating a further example of a method of controlling the medical instrument of fig. 1. The method shown in fig. 3 is similar to the method shown in fig. 2. The method steps in which a label is copied are equivalent steps. In this method, the steps of the method shown in fig. 2 have been modified. In this method, step 212 does not proceed directly to step 214. After performing step 212, the method proceeds to step 300. In step 300, a dynamic image is computed from a subset of all magnetic resonance data portions 152 characterized by late acquisition in the T1 relaxation curve. The magnetic resonance data portion is a trajectory in k-space, which is the portion of k-space in which magnetic resonance data for equilibrium magnetization has been sampled.

Next, in step 302, a decision block is used to query whether object motion above a predetermined threshold is detected using the balanced magnetization baseline image 156 and the dynamic image. This can be calculated, for example, from data acquired later in the T1 relaxation curve. If no motion is detected, the method returns from step 302 to step 214; if motion is detected, the method proceeds to step 303. Step 303 is an optional step between

steps

302 and 304. Step 304 is a delay in which the magnetic resonance imaging system is suspended to allow the magnetization to resume its equilibrium value. The pause may be, for example, at least 3 to 5 times the value of T1.

In step 304, the equilibrium magnetization magnetic resonance data 148 is reacquired by controlling the magnetic resonance imaging system using the first pulse sequence commands 140. In some instances, this may be beneficial to wait for the equilibrium magnetization to be restored within the object. This may require a delay of a few seconds, for example, the method may be paused for a period of around five seconds. Next, in step 306, the equilibrium magnetization baseline image 156 is recalculated from the equilibrium magnetization magnetic resonance imaging data 148 that has just been reacquired.

After step 306, the method proceeds directly to step 206, step 206 actually being to measure PRFS magnetic resonance data. After performing step 206, a re-calculation of the PRFS phase calculation is performed. There are a number of variations on how the PRFS phase calibration can actually be recalculated. Therefore, the recalculation of the PRFS phase calibration is not described in detail in fig. 3.

Fig. 4 shows a further example of a medical instrument 400. The medical instrument illustrated in fig. 4 is similar to the medical instrument illustrated in fig. 1, except that fig. 4 also includes a high intensity focused ultrasound system 402. The high intensity focused ultrasound system 402 is an example of a temperature control system for changing the temperature within the target zone 420.

Fig. 4 illustrates a further example of a medical instrument 400. The example shown in fig. 4 includes a temperature treatment system, which is a high intensity focused ultrasound system 402. The high intensity focused ultrasound system includes a fluid-filled chamber 404. Within the fluid-filled chamber 404 is an ultrasonic transducer 406. Although not shown in this figure, the ultrasound transducer 406 may include a plurality of ultrasound transducer elements, each capable of generating an individual beam of ultrasound. This may be used to electronically manipulate the position of sonication point 418 by controlling the phase and/or amplitude of the alternating current supplied to each of the ultrasound transducer elements.

The ultrasound transducer 406 is connected to a mechanism 408 that allows the ultrasound transducer 406 to be mechanically repositioned. The mechanism 408 is connected to a mechanical actuator 410 adapted to actuate the mechanism 408. The mechanical actuator 410 also represents a power source for supplying power to the ultrasound transducer 406. In some examples, the power supply may control the phase and/or amplitude of electrical power to individual ultrasound transducer elements. In some examples, the mechanical actuator/power source 410 is positioned outside the bore 104 of the magnet 102.

The ultrasound transducer 406 generates ultrasound, which is shown following a path 412. Ultrasound 412 travels through the fluid-filled cavity 224 and through the ultrasound window 414. In this example, the ultrasound then passes through the gel pad 416. The gel pad does not have to be present in all examples, but in this example there is a recess in the subject support 120 to accommodate the gel pad 416. The gel pad 416 helps to couple ultrasound power between the transducer 406 and the subject 118. After passing through the gel pad 416, the ultrasound 412 passes through the subject 118 and is focused to a sonication point 418. The sonication point 418 is focused within the target zone 420. The sonication point 418 can be moved by combining mechanically positioning the ultrasound transducer 406 and electronically steering the position of the sonication point 418 to treat the entire target zone 420. Such a medical instrument 400 may be used for treating tissue that is at least partially fat. Examples include, but are not limited to: breast tissue, tissue in the pelvic cavity, and tissue in the abdominal cavity.

The high intensity focused ultrasound system 402 is shown as also connected to the hardware interface 124 of the computer system 122. The contents of the computer system 122 and its storage 130 and memory 132 are equivalent to those shown in FIG. 1.

The computer storage 130 is shown as additionally containing a dynamic image 430 reconstructed from the magnetic resonance data portion 152. The computer storage 130 is also shown as containing temperature control system commands 432 that the processor 126 can use to control the high intensity focused ultrasound system 402.

The computer memory 132 is also shown as containing a motion detection module 440, the motion detection module 440 being capable of comparing the dynamic image 430 with the equilibrium magnetization baseline image 156 in order to detect motion of the object 118. The medical instrument 400 shown in fig. 4 and subsequent medical instruments shown in fig. 5 and 6 are also capable of performing the methods illustrated in fig. 2 and 3.

The computer memory 132 is also shown as containing a temperature control system command modification module 442, the temperature control system command modification module 442 being capable of modifying the temperature control system command 432 using the PRFS temperature map 162. Using the PRFS temperature map 162, the temperature control system command modification module 442 forms a closed control loop for controlling the high intensity focused ultrasound system 402. The software and control system described with respect to fig. 4 is generally applicable to other types of temperature control systems as well. It should be understood that in the following fig. 5 and 6, changes to the software may be made so that the software described in fig. 4 is also applicable to fig. 5 and 6.

Fig. 5 shows a further example of a medical instrument 500. The example shown in fig. 5 is similar to the example shown in fig. 4. The computer system 122 of fig. 5 is also equivalent to the computer system 122 shown in fig. 3 and 4. The contents of computer storage 130 and computer memory 132 are also equivalent to computer storage 130 and computer memory 132 shown in fig. 1, 3, and 4. In the example shown in fig. 5, a radio frequency tissue heating system 501 is used as the temperature treatment system. The radio frequency temperature treatment system 501 comprises an antenna 502 and a radio frequency transmitter 504. The antenna 502 is located near the target 420. Radio frequency energy generated by the transmitter 504 and radiated by the antenna 502 is used to selectively heat the target zone 420. In this example, the radio frequency transmitter 504 is shown connected to the hardware interface 124. The contents of the processor 126, as well as the computer storage 130 and computer memory 132, are used to control the radio frequency transmitter 504 in a manner equivalent to the way the processor 124 controls the high intensity focused ultrasound system 402 of figure 4.

Fig. 6 shows a further example of a medical instrument. In this example, a thermal treatment system 601 is shown. There is an applicator 602 that has been inserted into the subject 112. Near the end of the applicator 602 is a treatment zone 420. Here, the thermal treatment system 602 represents a general tissue heating system and may be, for example, a microwave or RF applicator, a cryoablator, or a laser. The applicator 602 may be adapted to supply microwave or RF energy for delivering hot, cryogenic substances to the subject 112, or may be adapted to deliver light into the target zone 420 to generate heat. Similarly, the supply system 604 may be a microwave or RF power supply, a supply system with a cryogenic or cooling fluid, or it may be a laser power supply. The thermal treatment system 601 is shown connected to the hardware interface 124 of the computer system 132. The contents of computer storage 130 and computer memory 132 are equivalent to the examples shown in fig. 1, 3, 4, and 5. The instructions and computer code contained therein allow the processor 124 to control the thermal treatment system 601 in a manner equivalent to the examples shown in fig. 4 and 5.

Fig. 7 shows a flow chart illustrating a method of operating the medical instrument of fig. 4, 5 or 6. The method shown in fig. 7 is similar to the method of fig. 3 with several modifications. The difference is that step 700 is performed before step 202, and in this example method, the method proceeds from step 202 to step 702, and then to step 204. The method may also proceed from step 218 to step 706 and then to step 220. First, in step 700, a temperature control system command is received. These commands may be received via a network connection, for example, or may be manually entered, such as by a physician or other operator. The method then proceeds to step 202 of fig. 3. After performing step 202, the method then proceeds to step 702. In step 702, the processor controls the temperature control system for zone 420. The method may also be performed without performing step 702. In this case, the method would proceed directly from step 202 to step 204. After performing step 702, the method proceeds to step 204 of FIG. 3. In step 218, a PRFS temperature map is calculated. Next, the method proceeds to step 706. In step 706, the temperature control system commands are modified using the PRFS temperature map 162. After the command has been altered, the method then proceeds normally to step 220 of FIG. 3.

PRFS temperature mapping is prior art during clinical MR-HIFU ablation, but for long sonication times as in hyperthermia, the PRFS temperature map is subject to errors due to B0 drift. A new acquisition and reconstruction for independent parallel T1-based temperature mapping is proposed to correct for such drift. It is based on interleaved T1 and PRFS sequences. The T1 sequence may be a saturation recovery Look-locker type sequence to reset the spin history from the previous PRFS. It is proposed to acquire the missing M0 information for the T1 reconstruction in a separate scan immediately before the start of the dynamic interleaved sequence (M0 scan) when the magnetization is still in equilibrium. It is proposed how to correct for motion that may occur between an M0 scan and a dynamic acquisition, as such motion will introduce errors in the pixel-by-pixel calculation of T1. Each of the most recent images acquired during T1 relaxation after saturation were compared to the M0 scan. The differences are evaluated by cross-correlation (deriving rigid body motion) or by elastic registration or by optical flow algorithms. If the motion exceeds a certain threshold since the M0 sweep, the dynamic sequence may stop, for example, by about 5 × T1 to allow an equilibrium magnetization to be established. Then, the M0 scan is repeated and dynamic interlaced imaging is started. HIFU sonication is stopped for an unsupervised period if clinically required. The T1 graph delivers independent temperature information, which is used to correct for B0 drift.

MR-guided high intensity focused ultrasound (MR-HIFU) is established as a new treatment option for various diseases, which concisely combines two non-invasive techniques. Treatment options include HIFU ablation and assisted HIFU hyperthermia — precisely controlled by MR temperature mapping to adjust the applied HIFU acoustic power and focal spot position in real-time. Currently, Proton Resonance Frequency Shift (PRFS) based temperature mapping is applied during clinical MR-HIFU treatment. HIFU hyperthermia requires long sonication times (> 20min) and parallel temperature mapping. PRFS-based temperature maps suffer from increasing errors over time because the unknown B0 offset B0(r) causes an outdated reference phase map after some time. The reference image cannot be re-acquired because the tissue has already been heated. This therefore facilitates deriving the drift by measuring the temperature independently, for example from a T1 diagram, and facilitates exploiting the known temperature dependence of T1.

Modern scanning software allows for fast interleaving of different imaging sequences with microsecond delays. For the above HIFU application, the dynamic T1 sequence should be interleaved with the PRFS sequence. The original version of the fast T1 mapping sequence followed T1 relaxation after inversion with small flip angle excitation to reach a minimized disturbance relaxation. The correction for this disturbance is known and can be applied.

The above Inversion Recovery (IR) based T1 mapping sequence can in principle be combined with k-space segmentation and interleaved with PRFS acquisition to derive a dynamic sequence. However, the IR-based approach does not work in such interleaved dynamic sequences, since the IR scheme requires the presence of the equilibrium magnetization M0 at the moment of inversion. Any previous PRFS acquisition disturbs this state. A known solution to this problem is to use a saturation recovery based variant in the interleaved scan that makes the subsequent T1 relaxation independent of the spin history (e.g., independent of the previous PRFS scan). However, such sequences are in principle not able to derive any information about M0, which is required for reconstruction for T1, for M0. This is solved by adding the saturation preparation as an extra pre-pulse followed by a latency and then adding the original IR prepared TFE-EPI sequence. This "clears" the spin history and leaves M0 information, but the latency with respect to 2 x T1 (the longest abdomen T1 is 1.5s) effectively doubles the overall acquisition time almost. This idea is therefore not applicable to dynamic interlaced scanning for HIFU.

New acquisition schemes and reconstructions are proposed which avoid additional acquisition time during the interlaced scan. The T1 sequence was based on pure saturation recovery followed by a Look-locker type sequence. It is dynamically interleaved with the standard PRFS sequence that acquires the same slices and potentially additional slices (see fig. 8 below). The T1 sequence is segmented such that after one saturation preparation, one of the M k-space segments is acquired at each time point ti after saturation. M T1 interlacesFilled together at different times t_iAt the N k-spaces acquired.

The information of the missing M0 can be acquired immediately in a separate scan before the start of the dynamic interleaved sequence (M0 scan). At this point, the magnetization is still in equilibrium. The M0 scan is proposed to be almost identical to the T1 interlace, however, there is no saturation preparation and a different k-space acquisition order to acquire the full image in one interlace. The low-high k-space order is used in case of the central k-space segment acquired in the first EPI run (see fig. 2). This ensures that the image contrast is mainly dominated by the equilibrium magnetization, while the excitation pulses used for the subsequent EPI-string/k-space segment have slightly disturbed the equilibrium. In subsequent T1 interleaving of the dynamic sequence, the flip angle, TR factor, TE factor, EPI factor, and other sequence parameters of the M0 scan should be chosen as those in the acquisition.

Fig. 8 illustrates a combined pulse sequence 800. The combined pulse sequence illustrates how the first pulse sequence command 140 is initially executed and then the second pulse sequence command 142 and the third pulse sequence command 144 are executed in an interleaved manner. During each of the executions of the third pulse sequence commands 144, the k-space trajectories for each time are separated so that virtually the entire set of magnetic resonance data portions is acquired and dynamic T1 magnetic resonance data can be reconstructed. A plot 802 illustrates acquisition of dynamic T1 magnetic resonance data. This is a timing diagram showing that radio frequency saturation preparation 804 is initially performed. This causes the longitudinal magnetization 806 to be 0. Over time, the magnetization 806 can be seen to recover. During each localization of dynamic T1 magnetic resonance data, plot 806 illustrates a first radio frequency pulse used to indicate when data was acquired, and block 808 illustrates a time window when data was acquired (see image 902) according to fig. 9. Figure 9 shows a k-space order of the balanced magnetization magnetic resonance data 900 and a k-space order 902 of the magnetic resonance data portion of the dynamic T1 magnetic resonance data. This illustrates the difference in data acquisition between the first pulse sequence 140 and the third pulse sequence command 144. During each of the executions 144, only a portion of k-space is acquired, as the data should be confined to a particular time window 808 along the relaxation curve. It should be noted, however, that k-space data is acquired along a certain trajectory, which enables both the central and outer regions of k-space to be sampled. After collecting several such portions, a new image representing the object at a later time along the relaxation curve can be calculated and used for motion detection. This provides a means to detect motion in addition to calibrating the PRFS temperature measurement.

Method for performing T1 reconstruction:

first, the M0 scan can be reconstructed by standard reconstruction and the signal proportional to M0 (except for the same factors as the late acquisition as the T2 relaxation term) is used to provide an undisturbed image.

During the dynamic phase, a complete set of M dynamic T1 interlaces (characterized by the fact that the entire k-space is covered for all N time points sampled on the relaxation curve) is used to reconstruct a series of N images with the effective acquisition time ti (i ═ 1 … N) during the relaxation after saturation preparation.

The pixel-by-pixel three-parameter fit [ ] is used to estimate the parameters M (0), M0, and T1 according to:

the apparent T1 is shorter than T1, and T1 can be calculated by:

for each pixel, among others, a respective pixel value in the image of the M0 scan for M0 is proposed. M (0) is here used as a fitting parameter to account for imperfections in saturation preparation, which may result in non-zero initial magnetization. M (0) can be assumed to be zero, otherwise a two parameter fit will result.

The strategy of collecting M0 information only once before dynamic collection raises issues of dealing with motion that may occur between M0 collection and dynamic collection. Such motion will introduce errors in the pixel-by-pixel calculation of T1.

Therefore, it is proposed to check whether the M0 image has become obsolete during dynamic acquisition, depending on the motion, as follows: each dynamic image M (n) reconstructed from the last point in time of relaxation is expected to be very similar in contrast to the M0 scan. Images M (n) and M0 are evaluated to derive a displacement field describing in-plane motion that has occurred since the M0 scan. The evaluation is proposed as a simple cross-correlation (deriving rigid body motion), elastic registration, or optical flow algorithm. Alternatively, the prior art similarity measure may be used to derive the similarity between M (n) and M0. The M0 scan is outdated if the motion since the M0 scan exceeds a user-defined threshold or the similarity between M (n) and M0 falls below a user-defined threshold. Therefore, the dynamic sequence has to be stopped for about 5 × T1, i.e. about 5s, to allow the establishment of equilibrium magnetization. Then, the M0 scan is repeated and dynamic imaging is started. HIFU sonication must be stopped for an unsupervised period if clinically required.

The temporal resolution of the dynamic series T1 images can also be increased by using a sliding window approach: the T1 interlace for each acquisition (providing a new segment of k-space lines for each time point ti) reconstructs a new T1 map with the latest set of M interlaces, effectively replacing the respective outdated set of k-space lines.

Example parameters for some pulse sequences are:

t1 sequence:

non-self (non-sel) SR ready T1 w-TFE; TFE excitation M ═ 5; TFE factor ═ 20; SENSE-P ═ 1.8; FOV 250 × 250mm 2; resolution is 1.42 × 1.42mm 2; the slice thickness is 4 mm; each interleaved Tacq 2000ms with N12 time points along the relaxation

PRFS sequence:

M2D T1w-FFE-EPI, TR/TE 41/19.5 ms; the flip angle is 19.5 degrees; EPI factor 7; SENSE-P ═ 1.8; FOV 250 × 250mm 2; resolution is 1.42 × 1.42mm 2; 3 slices; NSA ═ 2; fat suppression; dynamic acquisition time is 5.4s

Correction of PRFS temperature map:

with the known temperature dependence of T1, an independent temperature map of slice 2 is computed after each M T1 interlaces (or even after each interlace in the case of sliding window reconstruction) (see fig. 1). This temperature map is compared to a map derived from a dynamic PRFS sequence, which is reconstructed according to the prior art. The difference between the two temperature maps due to the B0 drift B0(r) is used to correct the PRFS temperature map by setting the respective reference phases for the PRFS reconstruction as known in the art.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. Although some measures are recited in mutually different dependent claims, this does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems. Any reference signs in the claims shall not be construed as limiting the scope.

Claims

1. A medical instrument (100, 400, 500, 600) comprising:

a magnetic resonance imaging system (102) for acquiring magnetic resonance data (148, 150, 152, 154) within an imaging zone (108);

a memory (132) storing machine executable instructions (170, 172, 174, 176, 440, 442), first pulse sequence commands (140), second pulse sequence commands (142), and third pulse sequence commands (144), wherein the first pulse sequence commands cause the magnetic resonance imaging system to acquire equilibrium magnetization magnetic resonance imaging data according to a T1 measurement magnetic resonance imaging protocol, wherein the second pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic Proton Resonance Frequency Shift (PRFS) magnetic resonance data (150) according to a proton resonance frequency shift magnetic resonance imaging protocol, wherein the third pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic T1 magnetic resonance data (150) according to the T1 measurement magnetic resonance imaging protocol, wherein the third pulse sequence commands further cause the magnetic resonance imaging system to sequentially acquire the dynamic T1 magnetic resonance data, as a set of magnetic resonance data portions (152), wherein the third pulse sequence commands cause the magnetic resonance imaging system to perform saturation preparation (804) at the beginning of the acquisition of each magnetic resonance data portion of the set of magnetic resonance data portions;

a processor (126) for controlling the medical instrument by executing the machine executable instructions, wherein execution of the machine executable instructions causes the processor to:

acquire (202) the equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands;

computing (204) a balanced magnetization baseline image from the balanced magnetization magnetic resonance imaging data;

wherein execution of the machine-executable instructions causes the processor to repeatedly:

acquiring (206) the dynamic proton resonance frequency shifted magnetic resonance data by controlling the magnetic resonance imaging system with the second pulse sequence commands;

acquiring (208) the set of magnetic resonance data portions (152) by controlling the magnetic resonance imaging system with the third pulse sequence commands, wherein the acquisition of the dynamic proton resonance frequency shifted magnetic resonance data is interleaved with the acquisition of the set of magnetic resonance data portions, wherein each magnetic resonance data portion of the set of magnetic resonance data portions corresponds to a different portion of k-space of the T1 measurement magnetic resonance imaging protocol and the set of magnetic resonance data portions covers the entire k-space of the T1 measurement magnetic resonance imaging protocol;

reassembling (212) the set of magnetic resonance data portions into the dynamic T1 magnetic resonance data after acquiring a complete set of magnetic resonance data portions;

calculating (214) a T1 map (158) using the reassembled dynamic T1 magnetic resonance data and an equilibrium magnetization image, wherein the equilibrium magnetization image is reconstructed from data acquired before the dynamic proton resonance frequency-shifted magnetic resonance data was acquired;

calculating (216) a proton resonance frequency shift phase calibration (160) using the dynamic proton resonance frequency shift magnetic resonance data and the T1 map; and is

Calculating (218) a proton resonance frequency-shift temperature map (162) using the dynamic proton resonance frequency-shift magnetic resonance data and the proton resonance frequency-shift phase calibration, if the proton resonance frequency-shift phase calibration has been calculated.

2. The medical instrument of claim 1, wherein execution of the instructions further causes the processor to repeatedly:

computing (300) a dynamic image (430) from the magnetic resonance data portion;

detecting (302) object motion above a predetermined threshold using the balanced magnetization baseline image and the dynamic image,

reacquiring (304) the equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands in case motion of the object is detected,

recalculating (306) the equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data if the subject motion is detected.

3. The medical instrument of claim 2, wherein the object motion is detected using any one of: cross-correlation algorithms, rigid motion detection algorithms, elastic registration algorithms, optical flow algorithms, and combinations thereof.

4. The medical instrument of claim 1, 2 or 3, wherein the medical instrument further comprises a temperature control system (402, 501, 601) for modifying a temperature within a target zone (420), wherein the target zone is within the imaging zone.

5. The medical instrument of claim 4, wherein the temperature control system is any one of: a high intensity focused ultrasound system (402), a radio frequency tissue heating system (501), a microwave applicator (601), a cryoablator (601), and a laser (601).

6. The medical instrument of claim 4, wherein execution of the machine executable instructions further causes the processor to:

receiving (700) a temperature control system command (432) that causes the temperature control system to alter the temperature of the target zone, and

repeatedly (706) altering the temperature control system commands using the proton resonance frequency shift temperature map.

7. The medical instrument of claim 6, wherein the medical instrument further comprises a user interface having a display, wherein execution of the machine-executable instructions further causes the processor to:

displaying the proton resonance frequency shift temperature map on the display;

receiving user control data from the user interface; and is

Modifying the temperature control system command using the user control data.

8. The medical instrument of claim 6, wherein execution of the machine executable instructions causes the processor to control (702) the temperature control system with the temperature control system commands.

9. The medical instrument of any one of claims 1-3, wherein the T1 measurement magnetic resonance imaging protocol is a saturation recovery hook-locker magnetic resonance imaging protocol.

10. The medical instrument of claim 2 or 3, wherein operation of the magnetic resonance imaging system causes the processor to perform the following at a predetermined time interval after acquiring the equilibrium magnetization magnetic resonance imaging data:

reacquiring the equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands, and

recalculating the equilibrium magnetization baseline image from the equilibrium magnetization magnetic resonance imaging data if the subject motion is detected.

11. A method of operating a medical instrument (100, 400, 500, 600), wherein the medical instrument comprises a magnetic resonance imaging system (102) for acquiring magnetic resonance data (148, 150, 152, 154) within an imaging zone (108); wherein the method comprises the steps of:

acquire (202) equilibrium magnetization magnetic resonance imaging data (148) by controlling the magnetic resonance imaging system using first pulse sequence commands (140), wherein the first pulse sequence commands cause the magnetic resonance imaging system to acquire the equilibrium magnetization magnetic resonance imaging data according to a T1 measurement magnetic resonance imaging protocol;

computing (204) a balanced magnetization baseline image (156) from the balanced magnetization magnetic resonance imaging data;

the method further comprises repeatedly:

acquiring (206) dynamic Proton Resonance Frequency Shift (PRFS) magnetic resonance data (150) by controlling the magnetic resonance imaging system with second pulse sequence commands (142), wherein the second pulse sequence commands cause the magnetic resonance imaging system to acquire the dynamic proton resonance frequency shift magnetic resonance data according to a proton resonance frequency shift magnetic resonance imaging protocol;

acquire (208) a set of magnetic resonance data portions (152) by controlling the magnetic resonance imaging system with third pulse sequence commands (144), wherein the third pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic T1 magnetic resonance data in accordance with the T1 measurement magnetic resonance imaging protocol, wherein the third pulse sequence commands further cause the magnetic resonance imaging system to sequentially acquire the dynamic T1 magnetic resonance data as the set of magnetic resonance data portions (152), wherein the acquiring of the dynamic proton resonance frequency shifted magnetic resonance data is interleaved with the acquiring of the set of magnetic resonance data portions, wherein each of the set of magnetic resonance data portions corresponds to a different portion of k-space of the T1 measurement magnetic resonance imaging protocol and the set of magnetic resonance data portions covers the entire k-space of the T1 measurement magnetic resonance imaging protocol, wherein the third pulse sequence commands cause the magnetic resonance imaging system to perform saturation preparation (804) at the beginning of the acquisition of each magnetic resonance data portion of the set of magnetic resonance data portions;

12. The method of claim 11, wherein the method further comprises repeatedly:

computing (300) a dynamic image (430) from selected magnetic resonance data selected from the set of magnetic resonance data portions, wherein the selected magnetic resonance data portions are selected to maximize longitudinal magnetization;

detecting (302) object motion above a predetermined threshold using the balanced magnetization baseline image and the dynamic image;

reacquiring (304) the equilibrium magnetization magnetic resonance imaging data by controlling the magnetic resonance imaging system using the first pulse sequence commands if the subject motion is detected;

13. The method of claim 12, wherein the object motion is detected using any one of: cross-correlation algorithms, rigid motion detection algorithms, elastic registration algorithms, optical flow algorithms, and combinations thereof.

14. A computer readable medium storing a computer program comprising machine executable instructions (170, 172, 174, 176, 440, 442) for execution by a processor (126) controlling a medical instrument (100, 400, 500, 600), wherein the medical instrument comprises a magnetic resonance imaging system (102) for acquiring magnetic resonance data within an imaging zone (108), wherein execution of the machine executable instructions causes the processor to:

acquiring (208) a set of magnetic resonance data portions (152) by controlling the magnetic resonance imaging system with third pulse sequence commands (144), wherein the third pulse sequence commands cause the magnetic resonance imaging system to acquire dynamic T1 magnetic resonance data (154) in accordance with the T1 measurement magnetic resonance imaging protocol, wherein the third pulse sequence commands further cause the magnetic resonance imaging system to sequentially acquire the dynamic T1 magnetic resonance data as the set of magnetic resonance data portions (152), wherein the acquiring of the dynamic proton resonance frequency shift magnetic resonance data is interleaved with the acquiring of the set of magnetic resonance data portions, wherein each of the set of magnetic resonance data portions corresponds to a different portion of k-space of the T1 measurement magnetic resonance imaging protocol and the set of magnetic resonance data portions covers the entire k-space of the T1 measurement magnetic resonance imaging protocol Wherein the third pulse sequence commands cause the magnetic resonance imaging system to perform saturation preparation (804) at the beginning of the acquisition of each magnetic resonance data portion of the set of magnetic resonance data portions;